Magnetic tunneling junctions with a magnetic barrier

ABSTRACT

Advanced magnetic tunneling junctions (MTJs) that dramatically reduce power consumption (switching energy, E SW ) while maintaining a reasonably high tunneling magnetoresistance (on/off ratio, TMR) and strong thermal stability at room temperature are described herein. The MTJs include a magnetic insulator, such as an antiferromagnetic material, as the tunnel barrier. A more energy efficient switching in the MTJs is achieved by magnon assisted switching.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional and claims benefit of U.S. applicationSer. No. 17/051,680 filed Oct. 29, 2020, which is a 371 and claimsbenefit of PCT/US2019/030932 filed May 6, 2019, which claims benefit ofU.S. Patent Application No. 62/667,380 filed May 4, 2018 and U.S. PatentApplication No. 62/812,809 filed Mar. 1, 2019, the specification(s) ofwhich is/are incorporated herein in their entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant Nos. 1554011and 1708180, awarded by NSF. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to tunneling junctions with a magneticinsulator as a tunnel barrier.

Background Art

New developments in the information age, such as big data, artificialintelligence, internet of things (IoT) and 5G wireless communications,directly rely on the continuous increase of computation power andexpansion of information technology infrastructure. Recent studiespredict that the global electricity usage of information & communicationtechnology (computers, data-centers, wireless network etc.) will morethan double in the next decade, which could account for 15% of totalelectricity produced. The dramatic increase of power consumption in partstems from a perennial demand for greater computation performance andincreased stand-by power dissipation of all complementary metal-oxidesemiconductor transistors when gate size is reduced to a few nanometers.

Spintronics represents a promising solution to address the problem ofgreatly increased power consumption in CMOS transistors for memory andlogic applications. Storing information as spin, instead of charge,spintronics offers a unique route to eliminate energy waste in thestand-by state. A large part of the present spintronics research focuseson reducing the dynamic power consumption (switching energy, E_(SW)) andincreasing the on/off ratio (magnetoresistance) in various structures.Magnetic tunnel junctions (MTJs) have been arguably the most importantbuilding blocks in spintronic technology. In most cases, the active partof a spintronic device is made of ferromagnetic (FM) materials formingan FM layer in the MTJ. Since the discovery of large tunnelmagnetoresistance (TMR) in MgO-based MTJs, research and development onMTJs have almost exclusively focused on MgO barriers. Indeed, MgO tunnelbarriers have shown tremendous advantages over other insulating barrierssuch as amorphous Al₂O₃. As known to one of ordinary skill in the art,the TMR is defined by: TMR=(R_(AP)−R_(P))/R_(P), where R_(P) and R_(AP)are the resistance of the MTJ in parallel and antiparallelconfiguration. The superior epitaxial growth of a MgO barrier withtransition metal ferromagnet electrodes makes the tunnel resistancerather tunable to meet the different requirement of specific devices,e.g., magnetic reading heads and magnetic random access memories(MRAMs). Most importantly, the TMR of MgO-based MTJs is as large as 600%at room temperature, far exceeding other known tunnel barriers.

While the large TMR value of MgO-based MTJs provides unprecedentedefficiency for magnetic reading, switching the magnetization directionof MTJs for writing remains challenging. In the first generation of MRAMdevices, an external magnetic field is used for magnetization switching;this method is not scalable and would fail for high density MRAMs. Thesecond generation takes advantage of spin-transfer torques (STTs) wherea sufficient large electric current across the tunnel barrier canreorient the relative magnetizations of two magnetic layers in parallelor antiparallel, depending on the polarity of the current. Up until now,the critical switching current density (j_(c)) for STT is very high, onthe order of 10⁶ A cm⁻². In STT switching, the spin angular momentum oftunnel electrons from one electrode to the other determines the totalmagnetic torque. Under a typical switching voltage across the junctionof about 0.5 V, each tunneling electron transfers its spin angularmomentum at a maximum of ℏ/2, where ℏ is Planck's constant, but theaccompanied energy of 0.5 eV is completely wasted. Thus, the STTswitching by tunnel electrons are not energy efficient. Hence, thereexists a need for a completely different MTJ in which energy efficiencyof STT switching is improved.

One alternative for STT switching is based on spin-orbit torques (SOTs).Instead of applying a switching current across the tunnel barrier, anin-plane current is applied to a heavy metal (HM) layer next to the freeFM layer. In the presence of the spin orbit coupling in the HM and atthe interface, the perpendicular spin currents provide the angularmomentum needed for the magnetization switching. While the above spinorbit torque is energetically favorable in theory (compared to thedirect STT torque) and multiple MTJs on a single HM wire can beswitched, the switching current remains large, of the order of10⁶-10⁷/cm² as well.

During the early stage of MTJ development, a Ni/NiO/Co junction wasreported to have small TMR at low temperature. When the Al₂O₃-based MTJwith more than 10% room temperature TMR was discovered in 1995, manyexperimental groups were racing to find better MTJs with a larger TMRvalue. After the MgO-based MTJs were discovered in 2004, the search fornew tunnel junctions was no longer of interest. Research effort has beenfocused on optimizing MgO-based MTJs which became a standard materialchoice for all spintronics applications. This present invention focuseson a different MTJ in which the barrier is an antiferromagneticmaterial.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide magnetic tunneljunctions (MTJs) with a magnetic insulator as a tunnel barrier, asspecified in the independent claims. It is a further objective of thepresent invention to provide MTJs with reduced E_(sw) while maintaininga large tunneling magnetoresistance (TMR) and strong thermal stabilityat RT. Embodiments of the invention are given in the dependent claims.Embodiments of the present invention can be freely combined with eachother if they are not mutually exclusive.

According to some aspects, the present invention features a tunneljunction comprising a first ferromagnetic layer, a second ferromagneticlayer, and a tunnel barrier juxtaposed between the first ferromagneticlayer and the second ferromagnetic layer. In one embodiment, the firstferromagnetic layer can have a fixed magnetization and the secondferromagnetic layer can have a free magnetization. Preferably, thetunnel barrier comprises a magnetic insulator layer that is magneticallyuncoupled to the free second ferromagnetic layer. In some embodiments,the magnetic order of the tunnel barrier can be of antiferromagnetic,ferrimagnetic, paramagnetic, or ferromagnetic nature.

For demonstration purposes, a non-limiting example of anantiferromagnetic insulator (AFI) will be described herein.Antiferromagnets can have a number of advantages compared with their FMcounterparts. For instance, the AF material have no net magnetizationtherefore AF cells can be packed into extremely high density withoutaffecting each other; and for the same reason, they are immune toexternal magnetic fields. Also, due to the staggered arrangement ofspins in AFs, the spin currents can penetrate much deeper into AFs.Furthermore, the magnetization switching frequency of AFs can be as highas THz, promising ultra-high speed and ultra-low energy operations.

In some embodiments, the present invention features a two-terminalAF-MTJ with a magnetoelectric (ME) Cr₂O₃ barrier. In other embodiments,the present invention utilizes other transitional metal oxides as an AFIreplacing MgO as the tunnel barrier. Without wishing to limit theinvention to any theory or mechanism, it is believed an AFI-based MTJcould significantly reduce the critical switching current densitycompared to nonmagnetic barrier-based MTJ, particularly, the MgO-basedMTJ. In preferred embodiments, the MTJs of the present inventionfeatures an AFI barrier that displays a large tunnel magnetoresistance(TMR) and other technologically friendly parameters such as tunablemagnetic anisotropy, favorable temperature and bias dependence of TMR,and high transparency for magnons to propagate across.

In one embodiment, the low energy switching can be achieved by themagnetoelectric effect. Due to a roughness-insensitive surfacemagnetization and a strong ME effect in oxides such as Cr₂O₃, thedirection of exchange bias (EB) to the free FM layer of the MTJ can beisothermally switched by a small electric field applied across theinsulator, which allows for manipulation of the magnetization of FM withsmall energy. In another embodiment, the low energy switching can beachieved by magnon assisted switching. When a finite voltage bias isapplied to the MTJ, the energy relaxation of the tunnel electrons leadsto asymmetric heating of two metallic layers. Consequently, there ismagnon current flowing across the magnetic insulator layer, resulting ina magnon transfer torque in addition to the electron spin-transfertorque. Compared to a tunnel junction with a nonmagnetic insulator withnegligible magnon transmission, the magnon transfer torque with a tunnelbarrier having magnetic order could be many times larger than theconventional spin-transfer torque of the tunnel electrons. This canprovide a more energy efficient switching in tunnel junctions.

In some embodiments, the tunnel junctions described herein may be usedfor information storage applications, stand-alone memories, logic unitsand oscillators. In other aspects, the tunnels junctions are suitablefor use in other spintronic devices such as lateral spin valves andneuromorphic processors for deep learning, as well as for emergenttechnologies such as wearable computers and Internet of Things, wherethe nonvolatile feature of spintronics is highly appreciated.

Any feature or combination of features described herein are includedwithin the scope of the present invention provided that the featuresincluded in any such combination are not mutually inconsistent as willbe apparent from the context, this specification, and the knowledge ofone of ordinary skill in the art. Additional advantages and aspects ofthe present invention are apparent in the following detailed descriptionand claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The features and advantages of the present invention will becomeapparent from a consideration of the following detailed descriptionpresented in connection with the accompanying drawings in which:

FIG. 1A is a non-limiting embodiment of a 2-terminal, 2-level structureof an antiferromagnetic magnetic tunneling junction (AFI-MTJ) of thepresent invention.

FIG. 1B is an example of a conventional 3-terminal, 3-level structure inthe prior arts.

FIG. 2 shows a non-limiting schematic of electrons in the AFI-MTJ. TwoFM layers are separated by an AFI barrier and they are biased by anegative voltage about 0.2-2V. Hot electrons tunnel from the left FMelectrode to the right FM electrode and the excess energy is dissipatedover inelastic scattering length to heat up magnons on the right. Theresulting magnons would diffuse from right (hot) to left (cold) mediatedby the magnons in AFI.

FIG. 3A illustrates a non-limiting embodiment of the AFI-MTJ structure.

FIG. 3B shows temperature profiles for both directions of the current inthe AFI-MTJ structure. The red solid line denotes electrons tunnel fromleft to right, and the blue solid line denotes electrons tunnel fromright to left.

FIG. 4 shows the dependence of the ratio of the switching currentdensity with and without the magnon spin torques on tunnel resistancearea product (RA) at different interface exchange coupling strength.

FIG. 5A shows a room temperature TMR curve for a Cr₂O₃-MTJ with in-planemagnetic anisotropy. FIG. 5B is a room temperature TMR curve for aCr₂O₃-MTJ with perpendicular magnetic anisotropy (PMA). FIG. 5C shows alow temperature TMR curve for the AF-MTJ shown in FIG. 5B. The exchangebias effect to the hard FM layer is fully controlled by the direction ofmagnetic field applied during cooling.

DETAILED DESCRIPTION OF THE INVENTION

Following is a list of elements corresponding to a particular elementreferred to herein:

-   -   100: tunneling junction    -   101, 102: ferromagnetic layer    -   103: tunnel barrier    -   104: electrons    -   105: magnons    -   107, 108: nonmagnetic layer

As used herein, the term “ferromagnetic” (FM) refers to a magneticordering of a material wherein the spins within the magnetic domain of amaterial having the same orientation, e.g. aligned in parallel (↑↑). Asused herein, the term “antiferromagnetic” (AF) refers to a magneticordering of a material wherein neighboring spins have opposingorientations, e.g. aligned in antiparallel (↑↓), and the magneticmoments of the neighbors are equal, thus the net moment is zero. As usedherein, the term “ferrimagnetic” is another type of magnetic orderingsimilar to antiferromagnetic except the magnetic moments of theneighbors are not equal, thus resulting in a net moment (↑↓). As usedherein, the term “paramagnetic” is defined as a magnetic ordering of amaterial characterized by having a magnetization caused by therealignment of magnetic moments of unpaired electrons due to thepresence of an external magnetic field. When the external magnetic fieldis removed, the magnetization is zero.

As known to one of ordinary skill in the art, the Neel temperature,T_(N), is the temperature above which an antiferromagnetic materialbecomes paramagnetic. In other words, the thermal energy becomes largeenough such that the material behaves paramagnetically with the magneticmoments aligned with the applied magnetic field direction thereforeenhancing the overall magnetization of the material.

Referring to FIG. 1A, in one embodiment, the present invention featuresa magnetic tunneling junction (MTJ) (100) comprising a firstferromagnetic layer (101), a second ferromagnetic layer (102), and anantiferromagnetic layer (103) disposed between the first and secondferromagnetic layers (101, 102). In some embodiments, the first andsecond ferromagnetic layers (101, 102) comprise CoFeB. In otherembodiments, the antiferromagnetic layer (103) may comprise amagnetoelectric (ME) material that provides a switching of exchange bias(EB). For example, the antiferromagnetic layer (103) is Cr₂O₃, such asepitaxial Cr₂O₃. In yet other embodiments, the antiferromagnetic layer(103) may be doped with materials to enhance the AF orderingtemperature, such as B or Al for example. The antiferromagnetic layer(103) may be treated with post-growth thermal annealing.

In some embodiments, the antiferromagnetic layer (103) is a tunnelbarrier that provides tunneling magnetoresistance (TMR). When anelectric field is applied across the antiferromagnetic layer (103), adirection of EB to the first or second ferromagnetic layer is switched,thereby manipulating the magnetization of the first and secondferromagnetic layers (101, 102). In one embodiment, theantiferromagnetic layer (103) can have a thickness of about 0.8 nm toabout 3.5 nm. In another embodiment, the first and second ferromagneticlayers (101, 102) may each have a thickness greater than 2 nm and theMTJ (100) exhibits an in-plane magnetic anisotropy. In yet anotherembodiment, the first and second ferromagnetic layers (101, 102) mayeach have a thickness of about 1 nm and the MTJ (100) exhibits aperpendicular magnetic anisotropy.

Due to a roughness-insensitive surface magnetization and a strong MEeffect of Cr₂O₃, the direction of exchange bias (EB) to an FM layer canbe isothermally switched by a small electric field (EF) applied acrossthe insulator, which allows for manipulation of the magnetization of FMwith ultra-low energy. A net magnetization (δM) can be developed in theentire volume of the film if the material is an insulator with a strongME coupling. The δM in Cr₂O₃ is strongly coupled to the AF orderparameter and its sign is controlled by the polarity of EF. Thereforeunder a constant magnetic field, the order parameter of the Cr₂O₃ can beswitched by the applied EF, leading to the reversal of the uncompensatedsurface magnetization, which subsequently switches the EB direction ofthe pinned FM. In some embodiments, the free layer of the MTJ can beswitched by voltage applied to the Cr₂O₃. Due to the small EF requiredto switch the AF order parameter, a much smaller E_(SW) is needed ascompared to other methods.

In the prior art, the lack of ultralow energy switching of resistancemay be partly due to a 3-terminal, 3-level geometry in previous designsof magnetoelectric MTJs, as shown in FIG. 1B, where the MTJ is grown ontop of (or below) a Cr₂O₃ layer. Two oxides are involved, one as the MEinsulator to switch the direction of exchange bias (EB) and the other asthe tunnel barrier. Two different voltage sources are required toconnect to the 3-level structure, one for ME switching and one for TMRreading, respectively. This approach has not yielded any successfulresult of voltage controlled MTJ, due to the difficulties in fabricatinga MTJ on top of the Cr₂O₃ and the challenges in 3-terminal, 3-levelstructures.

Instead of one ME oxide and one conventional tunnel barrier, someembodiments of the MTJ of the present invention utilizes a single Cr₂O₃layer as both tunnel barrier (providing TMR) and ME oxide (providing theswitching of EB). In a non-limiting example, of an in-plane MTJs, arobust TMR has been successfully obtained at RT, as shown in FIG. 5A.The Cr₂O₃ thickness of this particular MTJ is 3 nm. In MTJs withperpendicular magnetic anisotropy (PMA), a clear TMR curve at RT wasobtained as shown in FIG. 5B. The sharp transition of switching ischaracteristic of a sample with PMA. More importantly, the Cr₂O₃maintained the AF order at low temperature even at the small thicknessof ˜3 nm. An exchange bias effect to the magnetic hard layer can be seenin FIG. 5C. The direction of the EB is fully controlled by theorientation of the magnetic field during cooling. The blockingtemperature was found to be around 50K. The TMR at 11K is nearly 5 timeslarger than that of RT. This demonstrates that the AF order of thebarrier does not destroy TMR.

Without wishing to limit the present invention to a particular theory ormechanism, the results presented in FIGS. 5A and 5B demonstrates roomtemperature Cr₂O₃-MTJs with in-plane anisotropy and perpendicularanisotropy, respectively. More importantly, the antiferromagnetic natureof the Cr₂O₃ is directly revealed by the EB effect shown in FIG. 5C. Tobest of the inventors' knowledge, no TMR has been achieved at RT in MTJswith Cr₂O₃ barrier, nor any other AF oxide barrier where EB of the FMlayer has been observed in Cr₂O₃-MTJs.

Again, without wishing to limit the present invention, it is believedthat the TMR can be increased substantially by improving the dielectricproperties of the Cr₂O₃ barrier. This may be achieved by using afabrication method that grows a pinhole free, high density Cr₂O₃ barrierwith low roughness to increase the resistance of the barrier. In otherembodiments, the present invention utilizes a fabrication method thatgrows the MTJ with epitaxial Cr₂O₃ to substantially increase TMR, whichsubstantially increases with epitaxial Cr₂O₃. For instance, ultra-thinCr₂O₃ (0001) films can be grown on Co using a structure such asCo(0001)/Cr₂O₃(0001)/Co(0001), where the lattice mismatch between Co andCr₂O₃ is less than 2% (2a_(Co)=0.501 nm, a_(Cr2O3)=0.495 nm).

In some other embodiments, the Cr₂O₃ barriers may be fabricated withNeel temperature (T_(N)) above RT by doping the barrier. The T_(N) ofbulk Cr₂O₃ is 307K, which reduces in thin film samples. For theME-controlled MTJs, a barrier thickness in the range of 1-2 nm isdesired in some embodiments. Therefore, the T_(N) of Cr₂O₃ thin filmsmust be increased to well above 300K. In a non-limiting example of aCr₂O₃-MTJ, the T_(N) may be increased to above 300K, for example, to atleast 400K, with B or Al doping in Cr₂O₃. In some embodiments, theamount of B, or Al doping may range from about 1% to about 5%. Forinstance, the amount of doping ranges from about 2% to 3%.

In a non-limiting embodiment, the FM layers of the AF-MTJ were selectedto comprise CoFeB so that the Boron in the CoFeB alloy diffuses out theFM layer during post-growth thermal annealing. As an example, the CoFeBcomposition may be Co₂₀Fe₆₀B₂₀ for both FM layers. Without wishing to bebound by a particular mechanism, the crystallization of amorphous CoFeBduring thermal annealing diffuses out B atoms which are absorbed by boththe tunnel barrier and the heavy metal layer adjacent to the CoFeBlayer. In some embodiments, the T_(N) may be increased substantiallywith further annealing. Both the annealing temperature and annealingtime can be precisely tuned in a rapid thermal anneal procedure toachieve optimal doping of B in the Cr₂O₃ barrier. In other embodiments,doping with Al may also substantially enhance the blocking temperatureas well as the unidirectional anisotropy energy of Cr₂O₃. In anon-limiting example, the Al doped Cr₂O₃ can be fabricated byco-sputtering of Cr₂O₃ and Al. In yet other embodiments, Cr₂O₃ may bedoped with Fe.

According to some embodiments, the present invention features atunneling junction (100) comprising a first ferromagnetic layer (101)having a fixed magnetization, a second ferromagnetic layer (102) havinga free magnetization, and a tunnel barrier (103) disposed between thefirst and second ferromagnetic layers (101, 102). In a preferredembodiment, the tunnel barrier (103) comprises an insulator layer havinga magnetic order. The insulator layer is magnetically uncoupled to thefree second ferromagnetic layer (102).

Referring to FIG. 2 , when an electric field having a low voltage at thefirst ferromagnetic layer (101) and a high voltage at the secondferromagnetic layer (102) is applied across the tunnel barrier (103),electrons (104) tunnel from the first ferromagnetic layer to the secondferromagnetic layer. The electrons have a spin that carries angularmomentum, which generates a torque on the second ferromagnetic layer.The tunneling of the electrons generates heat in the secondferromagnetic layer. The heat generates a thermal gradient across thetunnel barrier (103) and excites magnons (105) that carry angularmomentum. The angular momentum carried by the magnons is opposite insign to the angular momentum of the tunneling electrons. Since thetunneling barrier (103) comprises the insulator layer with magneticorder, the thermal gradient causes a magnon current to flow across thetunnel barrier (103) from the second ferromagnetic layer (102) to thefirst ferromagnetic layer (101). Since the magnon current carriesmomentum of the opposite sign, the magnon current transfers angularmomentum having the same sign as the tunnel current to the secondferromagnetic layer (102). Thus, the angular momentum can change amagnetization direction of the second ferromagnetic layer (102).

In some embodiments, the magnetic order of the insulator layer can beantiferromagnetic, ferrimagnetic, ferromagnetic, or paramagnetic. In oneembodiment, the insulator layer may comprise an antiferromagneticmaterial such as Cr₂O₃, NiO, FeO, CoO, BiFeO₃, SrIrO₄, or otherantiferromagnetic materials. In another embodiment, the insulator layermay comprise a ferromagnetic material such as Fe, Co, Ni, Fe₃O₄,Sr-doped lanthanum manganite perovskites (La_(1-x)Sr_(x)MnO₃, where x=0,0.1, 0.15, 0.2, 0.3), Yttrium iron garnet (YIG), or other ferromagneticmaterials or other transition metal oxides. In yet another embodiment,the insulator layer may comprise a ferrimagnetic material such asferrites, magnetic garnets, or magnetite.

In some other embodiments, the insulator layer may comprise a materialthat is antiferromagnetic below the Neel temperature and paramagneticabove the Neel temperature, such as LaMnO₃ or CaMnO₃. In otherembodiments, the insulator layer may comprise a material that isferromagnetic below the Curie temperature and paramagnetic above theCurie temperature. For example, the insulator layer may compriseLaCoMnO₃, LaSrMnO₃, where the La concentration is ⅔ and the Co or Srconcentration is ⅓.

In some embodiments, the first ferromagnetic layer (101) may compriseFe, Co, Ni, Fe₃O₄, Sr-doped lanthanum manganite perovskites(La_(1-x)Sr_(x)MnO₃, where x=0, 0.1, 0.15, 0.2, 0.3), Yttrium irongarnet (YIG), or other ferromagnetic materials. The Sr-doped lanthanummanganite perovskites may be according to La_(1-x)Sr_(x)MnO₃, where xranges from 0-1. For example, x=0, 0.1, 0.15, 0.2, 0.3. In otherembodiments, the first ferromagnetic layer (101) may comprise acombination of ferromagnetic materials and nonmagnetic materials, suchas alloys of Co_(x)Fe_(y)B_(z), wherein x ranges from 0-95, y rangesfrom 0-95, and z ranges from 0-60.

In one embodiment, the second ferromagnetic layer (102) may comprise Fe,Co, Ni, Fe₃O₄, Sr-doped lanthanum manganite perovskites(La_(1-x)Sr_(x)MnO₃, wherein x ranges from 0-1), Yttrium iron garnet(YIG), or other ferromagnetic materials. In another embodiment, thesecond ferromagnetic layer (102) may comprise a combination offerromagnetic materials and nonmagnetic materials, such as alloys ofCo_(x)Fe_(y)B_(z), wherein x ranges from 0-95, y ranges from 0-95, and zranges from 0-60.

In further embodiments, the tunneling junction (100) may include a firstnonmagnetic layer (107) interfacing with one of the ferromagneticlayers. The tunnel junction (100) may further comprise a secondnonmagnetic layer (108) interfacing with the other ferromagnetic layersuch that the first ferromagnetic layer (101), the second ferromagneticlayer (102), and the tunnel barrier (103) are juxtaposed between thefirst nonmagnetic layer (107) and the second nonmagnetic layer (108).Examples of the nonmagnetic materials making up the nonmagnetic layersinclude, but are not limited to, Ru, Ta, Mo, W, Pt, Zr, Hf, oxides,nitrides, and combinations thereof.

According to some embodiments, the present invention features anon-volatile, magnetoresistive storage device comprising a non-volatilemagnetoresistive storage element. The storage element may comprise anyof the tunneling junctions described herein.

According to other embodiments, the present invention features aspintronic device comprising any of the tunneling junctions describedherein. Spintronics, or spin electronics, focuses on electron spin,instead of charge, for information processing and storage in solid statedevices. Information can be stored into spin as one of two possibleorientations. Spintronic devices can combine logic and storagefunctionality, thereby eliminating the need for separate components.Spintronics can be used in MRAM applications and semiconductortransistors for memory and logic applications. Non-limiting examples ofspintronic devices include magnetic hard disk drives, random accessmemories, spin logic cells, sensors, microwave oscillators, andamplifiers.

EXAMPLE

The following is a non-limiting example of the present invention,specifically, a magnetic tunneling junction with an antiferromagneticinsulator. It is to be understood that said example is not intended tolimit the present invention in any way. Equivalents or substitutes arewithin the scope of the present invention.

It is an objective of the invention to provide an MTJ with an AFIreplacing MgO as the tunnel barrier for tunnel transport. Consider atunnel junction made of two FM metals separated by a thin AFI, as shownin FIG. 3A. When a voltage is applied across the tunnel barrier,electrons tunnel from the electrode with the lower voltage to that withthe higher voltage. While the tunneling electron will relax its energyin both electrodes, the majority of the energy is relaxed in theelectrode receiving the tunnel electron. Since the inelastic mean freepath is only a few angstroms for the tunnel electron with the energyabout 0.5 eV above the Fermi level, the heat is generated near thevicinity of the interface. The heat subsequently diffuses into theinterior of the electrode as well as across the barrier. In the steadystate condition, a temperature gradient is established in the structureand a temperature difference would be created at the two sides of thebarrier. The temperature difference could reach a fraction of a Kelvindegree for a bias voltage of 0.5-1 V. Consequently, a magnon currentwould flow across the AFI barrier from one FM electrode to the other,exerting a magnon transfer torque on the free magnetization layer.

Without wishing to be bound to a particular theory or mechanism, it maybe possible to recycle the wasted energy of tunnel electrons formagnetization switching. Since a magnetic barrier is required for magnonpropagation, both FM and AF insulators may be barrier candidates.

Heat Transport and Temperature Profile

To model the temperature profile, examples of geometrical parameters arespecified in FIG. 3A: the MTJ, comprising a pinned magnetic layer FM1,an AFI barrier, and a free magnetic layer FM2, is sandwiched by twononmagnetic (NM) layers (representing the overlayer and underlayer ofMTJs) so that the temperature profile is not simply limited within theMTJ. Thicknesses of the layers are labeled in FIG. 3B. A time-dependentelectric current j_(e)(t) flows perpendicularly to the layers with abias voltage V(t) across the junction. The sign convention for thecurrent is j_(e)(t)<0 (or equivalently V(t)<0) corresponding to netelectron tunneling from FM1 to FM2. The current density used in FIG. 3Bis j_(e)=2×10⁶ A cm⁻² and the voltage is V=0.2 V. Other materialparameters include: d_(FM1)=2d_(FM1)=3d_(AF1)=3 nm, d_(N)=30 nm,λ_(inel)=1 nm, α=0.9, κ_(N)=401 W m⁻¹ K⁻¹, κ_(AF)=91 W m⁻¹ K⁻¹,κ_(AF)=20 W m⁻¹ K⁻¹, σ_(F)=1.43×10⁷ S m⁻¹, and σ_(N)=5.96×10⁷ S m⁻¹. Inan exemplary embodiment, the FM may be Ni, the AFI may be NiO, and theNM may be Cu. The temperatures at the outer boundaries of the MTJ may bekept at 300 K.

The heat transport can be modeled by using a layer-by-layer approach. Ineach layer, the heat diffusion equation reads:

$\begin{matrix}{{{\rho_{i}C_{i}\frac{\partial{T\left( {t,x} \right)}}{\partial t}} - {\kappa_{i}\frac{\partial^{2}{T\left( {t,x} \right)}}{\partial x^{2}}}} = {P_{i}\left( {t,x} \right)}} & (1)\end{matrix}$

where ρ_(i), C_(i), and κ_(i) are the mass density, heat capacity, andthermal conductivity of the i^(th) layer, and P_(i)(t, x) is the powerof heat source generated by the electric current. The Joule heatingj_(e) ²/σ_(i) is always present for each metallic layer where σ_(i) iselectric conductivity. In the tunnel junction, as illustrated in FIG. 2, the main energy relaxation of the tunnel electrons occurs near theinterface. For the electrode receiving the tunnel electrons, the energyof tunnel electrons is above the Fermi level up to the bias voltageV(t). These hot electrons have short mean free paths, on the order of 1nm. For the electrode emitting electrons, the holes left by the tunnelelectrons are also short lived and thus, the annihilation of holes takesplace near the interface as well.

Therefore, the heat generation by tunnel electrons may be parameterizedas:

$\begin{matrix}{{P_{re}\left( {t,x} \right)} = {\alpha\frac{{j_{e}(t)}{V(t)}}{\lambda_{inel}}\exp\exp\left( {- \frac{❘x❘}{\lambda_{inel}}} \right)}} & \left( {2a} \right)\end{matrix}$ $\begin{matrix}{{P_{em}\left( {t,x} \right)} = {\left( {1 - \alpha} \right)\frac{{j_{e}(t)}{V(t)}}{\lambda_{inel}}\exp{\exp\left( {- \frac{❘x❘}{\lambda_{inel}}} \right)}}} & \left( {2b} \right)\end{matrix}$

where |x| is the stack position from AFI/FM interface, a is toparametrize the relative heat power generated in two electrodes, andλ_(inel) is the inelastic scattering mean free path. The parameter α isalways larger than 0.5, i.e., the electron-receiving electrode generatesmore heat; this is because the tunnel probability is larger for tunnelelectrons with higher energy.

Since the characteristic time of magnetization dynamics (aboutpicoseconds) is much longer than the electron-electron andelectron-phonon collision times (about tens of femtoseconds) whichcontrol the rate of change of the temperature, the above heat diffusionis solved in the steady state condition, i.e., the source andtemperature are assumed to become constant once an electric current isturned on. Equation (1) becomes a simple differential form and thegeneral solutions can be found for each layer. The integration constantsare then determined by boundary conditions in which the continuity ofthe temperature and heat current across the interfaces are used.

FIG. 3B shows a temperature profile of a tunnel junction using thematerials parameters in bulk form. Since an asymmetric heating parameterα=0.9 is assumed, i.e., 90% of the Joule heating is generated at theelectron receiving electrode, the temperature is always higher for thehigh voltage side of the junction. The temperature difference acrosstunnel barrier could reach tens of milli-Kelvin for a current density ofj_(e)=2×10⁶ A cm⁻² and voltage of 0.2 V. The actual temperature gradientacross the barrier can be even larger when the stack structure and thepassivation materials used in the MTJ device are optimized.

Magnon Current and Magnon Transfer Torques

The magnon current in the presence of a temperature gradient in FM maybe written as:

j _(m) ^(F)(x)=−ℏS _(m)∇_(x) T(x){circumflex over (M)} _(F)−σ_(m)^(F)∇_(x)μ_(m)(x){circumflex over (M)} _(F)  (3)

where S_(m) is the spin Seebeck coefficient, and σ_(m) ^(F) is themagnon conductivity. The effective magnon chemical potential μ_(m)(x) isused to describe the nonequilibrium magnon accumulation, and {circumflexover (M)}_(F) is the FM magnetization. Within the AFI layer with twosublattices, the magnon Ohm's law is

j _(m) ^(AF)(x)=−σ_(m) ^(AF)∇_(x)μ_(m)(x)  (4)

where σ_(m) ^(AF) is the AFI magnon conductivity. Considering easy-axiscollinear AFI, two degenerate magnon branches cancel out thereforemagnon spin Seebeck effect in AFI is not considered.

The exchange interaction at AFI/FM interface is responsible for themagnon transmission:

H _(int) =−J _(int)Σ_(i) S _(i,F) ·S _(i,α(b))  (5)

where J_(int) is the interface exchange constant, S_(i,F) represents thespin at the interface of FM layer, and S_(i,α(b)) is the spin of twosublattices of AFI. Here, it is considered that (1) both FM and AFI havein-plane uniaxial anisotropy, and (2) the AFI interface is a compensatedone such that the exchange coupling between the ferromagnetic spin andeither sublattice spin of the AFI is modeled by the same J_(int). Theorder parameter of AFI is assumed to have an angle to the magnetizationof FM, thus the second quantization of Equation (5) would be:

H _(int) =−J _(int)Σ_(kq)(S _(F) S _(AF))^(1/2) [C _(q) A _(k)α_(q)^(†)(1+{circumflex over (n)}·{circumflex over (M)} _(F))+C _(q) A_(k)β_(q) ^(†)(1−{circumflex over (n)}·{circumflex over (M)}_(F))+H.c.]δ _(k,q)  (6)

where n is the AFI order parameter, A_(k) (A_(k)) represents theannihilation (creation) operators for the FM magnons, α_(q) ^(†), α_(q)and β_(q) ^(†), β_(q) are the creation and annihilation operators forthe two magnon branches of AFI, C_(q)=u_(q)−v_(q) where u_(q) and v_(q)are the Bogoliubov transformation coefficients of AFI magnons, S_(F(AF))is the magnitude of FM (AFI) spin, and the high order magnoninteractions have been neglected.

Two sets of boundary conditions at interfaces are needed to determinethe integration constants from Equations (3) and (4). The first one isthat the longitudinal magnon spin current is continuous across theFM/AFI interface,

j _(m) ^(F) ={circumflex over (M)} _(F) ·j _(m) ^(AF)  (7)

and their magnitude is related to the difference of magnon chemicalpotential at two sides of the interface:

j _(m) =G _(A/F) ^(∥)[μ_(m) ^(F)−μ_(m) ^(AF) ·{circumflex over (M)}_(F)]  (8)

where G_(A/F) ^(∥) is the longitudinal magnon spin conductance. For the{circumflex over (n)}·{circumflex over (M)}_(F)=1 case, the interfaceexchange interaction in the form of J_(int)A_(k)α_(q) ^(†) leads to aspin current across the interface. For temperature much lower than theCurie and Neel temperatures, the longitudinal magnon spin conductancescales with

$\frac{J_{int}^{2}}{\left( {k_{B}T_{C}} \right)\left( {k_{B}T_{N}} \right)}\left( \frac{T}{T_{C}} \right)^{1/2}{\left( \frac{T}{T_{N}} \right)^{2}.}$

For the case in which the quantization axis of AFI is perpendicular tolocal magnetization of FM, e.g., {circumflex over (n)}·{circumflex over(M)}_(F)=0, both α_(q) and β_(q) can create an FM magnon with theinteraction J_(int)A_(k) ^(†)(α_(q)+β_(q)). Since α_(q) and β_(q) haveopposite spin direction, the nonequal accumulations of these two magnonswould create a transverse spin torque on FM. The second boundarycondition would be:

{circumflex over (M)} _(F) ×[{circumflex over (M)} _(F) ×j _(m) ^(AF)]=−G _(A/F) ^(⊥) {circumflex over (M)} _(F) ×[{circumflex over (M)}_(F)×μ_(m) ^(AF)]  (9)

where G_(A/F) ^(⊥) is analogous to the mixing conductance and itsmagnitude is half the longitudinal one. After a detailed derivation (notshown), the magnon conductance is:

$\begin{matrix}{G_{{AF}/F}^{} = {{2G_{{AF}/F}^{\bot}} = {\frac{\pi S_{F}S_{AF}J_{int}^{2}a_{F}^{2}a_{AF}^{2}}{2k_{B}T}{\int{d\varepsilon_{q}d{\varepsilon_{q^{\prime}}\left( {u_{q} - v_{q}} \right)} \times {g_{m}^{F}\left( \varepsilon_{q} \right)}{g_{m}^{AF}\left( \varepsilon_{q} \right)} \times {csch}^{2}\frac{\varepsilon_{q}}{2k_{B}T}}}}}} & (10)\end{matrix}$

where a_(F(AF)) is the lattice constant of the FM(AFI) material andg_(m) is the density of states of magnon. Note that the magnon currentin FM layers is always parallel to the direction of the magnetization,as in the case of the electron spin current.

With these boundary conditions and the temperature profile, the magnonaccumulation and magnon current can be determined in each layer. Themagnon torque on the free layer FM2 was identified as the transversecomponent (relative to the magnetization vector of the FM2) of themagnon current at the AFI/FM2 interface.

Amplification of Spin Torques

To quantitatively estimate the enhancement of the spin torque by usingan AFI barrier, the magnon current was calculated and the magnon spintorque was obtained due to the temperature difference generated by thetunnel electrons.

The critical torque, τ_(cr), for the switching of the free layer ischosen to be equivalent to the critical electric current j_(cr)⁽⁰⁾=5×10⁶ A cm⁻² in the absence of the magnon spin torque. When themagnon spin torque is turned on, the new critical electric currentdensity j_(cr) ^((m)) needed to generate the same amount of torqueτ_(cr) is numerically determined. As the magnon torque is directlyrelated to the Joule heating, the relative contribution between magnoncurrent and electron spin current depends highly on the tunnelresistance. The larger the voltage (or the resistance), the greater themagnon torque relative to the electron spin torque.

FIG. 4 shows the dependence of the ratio of the switching currentdensity with and without the magnon spin torques on the tunnelresistance area product (RA) for σ=π/2 (the angle between themagnetization directions of the two magnetic layers) at differentinterface exchange coupling strengths. It is assumed that the criticalelectric current j_(cr) ⁽⁰⁾=5×10⁶ A cm⁻² with polarization P=0.5 in theabsence of the magnon spin torque. The RA value scales exponentiallywith the barrier thickness, RA=10 μm² at d_(AF1)=1 nm and RA=10³ μm² atd_(AF1)=2 nm. Other parameters used include: d_(FM1)=2d_(FM2)=3 nm,T_(C)=630K, T_(N)=530K, a_(F)=0.35 nm, a_(AF)=0.417 nm, S_(F)=S_(AF)=2.In FIG. 4 , all lines represent the exact same total torque τ_(cr). Asthe resistance and the exchange coupling increase, the magnon spintorque increases, and thus the electric current needed to generate thesame total torque reduces. Without wishing to be bound to a particulartheory or mechanism, a large tunnel resistance can generate a largermagnon spin torque and therefore a thicker tunnel barrier is favored. Athick barrier thickness usually improves the tunnel magnetoresistance.However, for device applications, the tunnel resistance has to matchwith other parts of the electronics and thus the resistance cannotincrease indefinitely.

The spin currents, or the angular momentum currents, of tunnel electronsand diffusive magnons, are not always additive. Consider themagnetization of two magnetic electrodes in parallel. If the majorityelectrons have a larger tunnel conductance than the minority electrons,the electron spin current would be additive to the magnon currentbecause the spin direction of magnons is always antiparallel to themajority electrons and the flow direction of magnons from asymmetrictunneling heating is opposite to the (spin) electron current. Thus, itis essential to choose a MTJ in which majority electron tunnelingdominates.

In conclusion, the present invention advantageously provides an AFIbarrier-based MTJ that can achieve a significantly lower switchingcurrent density by “reuse” of the wasted energy of tunnel electrons. Theadvantage of using an AFI over an NM barrier is that the AFI barrierprovides a magnon propagating gateway for additional spin-transfertorques created by the tunnel electrons induced thermal gradient. Insome embodiments, the AFI-based MTJ may be utilized in lowering theswitching current density of spin-transfer torque-based MRAMs.

Although there has been shown and described the preferred embodiment ofthe present invention, it will be readily apparent to those skilled inthe art that modifications may be made thereto which do not exceed thescope of the appended claims. Therefore, the scope of the invention isonly to be limited by the following claims. Reference numbers recited inthe below claims are solely for ease of examination of this patentapplication, and are exemplary, and are not intended in any way to limitthe scope of the claims to the particular features having thecorresponding reference numbers in the drawings. In some embodiments,the figures presented in this patent application are drawn to scale,including the angles, ratios of dimensions, etc. In some embodiments,the figures are representative only and the claims are not limited bythe dimensions of the figures. In some embodiments, descriptions of theinventions described herein using the phrase “comprising” includesembodiments that could be described as “consisting essentially of” or“consisting of”, and as such the written description requirement forclaiming one or more embodiments of the present invention using thephrase “consisting essentially of” or “consisting of” is met.

What is claimed is:
 1. A tunneling junction comprising: a. a firstferromagnetic layer having a fixed magnetization; b. a secondferromagnetic layer having a free magnetization; and c. a tunnel barrierjuxtaposed between the first ferromagnetic layer and the secondferromagnetic layer, the tunnel barrier comprising an insulator layerhaving a magnetic order, wherein the insulator layer is magneticallyuncoupled to the second ferromagnetic layer.
 2. The tunneling junctionof claim 1, wherein when an electric field having a low voltage at thefirst ferromagnetic layer and a high voltage at the second ferromagneticlayer is applied across the tunnel barrier, electrons tunnel from thefirst ferromagnetic layer to the second ferromagnetic layer, wherein theelectrons have a spin that carries angular momentum, wherein the angularmomentum generates a torque on the second ferromagnetic layer, whereinthe tunneling of the electrons generates heat in the secondferromagnetic layer, wherein the heat generates a thermal gradientacross the tunnel barrier, wherein the heat excites magnons that carryangular momentum, wherein the angular momentum carried by the magnons isopposite in sign to the angular momentum of the tunneling electrons,wherein because the tunnel barrier comprises the insulator layer withthe magnetic order, the thermal gradient causes a magnon current to flowacross the tunnel barrier from the second ferromagnetic layer to thefirst ferromagnetic layer, wherein because the magnon current carriesmomentum of an opposite sign, the magnon current transfers angularmomentum with a sign identical to a tunnel current to the secondferromagnetic layer, wherein the angular momentum can change amagnetization direction of the second ferromagnetic layer.
 3. Thetunneling junction of claim 1, wherein the magnetic order of theinsulator layer is antiferromagnetic, ferrimagnetic, ferromagnetic, orparamagnetic.
 4. The tunneling junction of claim 3, wherein theinsulator layer is antiferromagnetic below Neel temperature andparamagnetic above Neel temperature.
 5. The tunneling junction of claim4, wherein the insulator layer comprises LaMnO₃ or CaMnO₃.
 6. Thetunneling junction of claim 3, wherein the insulator layer isferromagnetic below Curie temperature and paramagnetic above Curietemperature.
 7. The tunneling junction of claim 6, wherein the insulatorlayer comprises LaCoMnO₃ or LaSrMnO₃, wherein La concentration is ⅔, andCo and Sr concentration are ⅓.
 8. The tunneling junction of claim 1,wherein the insulator layer comprises Cr₂O₃, NiO, FeO, CoO, BiFeO₃, orSrIrO₄.
 9. The tunneling junction of claim 1, wherein the insulatorlayer comprises Fe, Co, Ni, Fe₃O₄, Yttrium iron garnet (YIG),La_(1-x)Sr_(x)MnO₃, wherein x ranges from 0-1, or other transition metaloxides.
 10. The tunneling junction of claim 1, wherein the insulatorlayer comprises a combination of ferromagnetic materials and nonmagneticmaterials.
 11. The tunneling junction of claim 10, wherein thenonmagnetic materials comprise Ru, Ta, Mo, W, Pt, Zr, Hf, oxides,nitrides, or combinations thereof.
 12. The tunneling junction of claim1, wherein the first ferromagnetic layer and the second ferromagneticlayer are each independently comprised of Fe, Co, Ni, Fe₃O₄, Yttriumiron garnet (YIG), or La_(1-x)Sr_(x)MnO₃, wherein x ranges from 0-1. 13.The tunneling junction of claim 1, wherein the first ferromagnetic layerand the second ferromagnetic layer are each independently comprised of acombination of ferromagnetic materials and nonmagnetic materials. 14.The tunneling junction of claim 1, wherein the first ferromagnetic layerand the second ferromagnetic layer are each independently comprised ofalloys of Co_(x)Fe_(y)B_(z), wherein x ranges from 0-95, y ranges from0-95, and z ranges from 0-60.
 15. The tunneling junction of claim 1,further comprising a first nonmagnetic layer interfacing with one of theferromagnetic layers.
 16. The tunneling junction of claim 15 furthercomprising a second nonmagnetic layer interfacing with the ferromagneticlayer such that the first ferromagnetic layer, the second ferromagneticlayer, and the tunnel barrier are juxtaposed between the firstnonmagnetic layer and the second nonmagnetic layer.
 17. A non-volatile,magnetoresistive storage device comprising a non-volatilemagneto-resistive storage element comprising a tunneling junctionaccording to claim
 1. 18. A spintronic device comprising a tunnelingjunction according to claim
 1. 19. The spintronic device of claim 18,wherein the device is a magnetoresistive random access memory (MRAM)device, a spin logic cell, a sensor, a microwave oscillator, or anamplifier.